1. Field of the Invention
This invention relates generally to brushgear, and more particularly to brushgear having brushes, each comprising a terminal and a brush arm, made of electrically conductive metal strips, the brushes being held in position by being inserted into terminal holes provided on a side plate of a motor case, made of a synthetic resin, in which the brushes are held securely in position by mechanically joining the terminal and the brush arm of each brush, which are formed of electrically conductive metal strips as independent components, into one piece.
2. Description of the Prior Art
In an electric motor, in general, it is necessary to keep the brushes and the commutator thereof in an excellent contact state.
FIGS. 1 through 7 are diagrams of assistance in explaining the prior art of brushgear.
In FIGS. 1 through 7, reference numeral 1 refers to a brush arm; 2 to a brush base; 3 to a terminal; 4 to a motor case side plate; 5 to a first support member; 6 to a second support member; 7 to a commutator; 8 to a reinforcement; 9 to an auxiliary support means; 10 and 10' to bent portions; 11 to a reinforcing portion; 12 to a terminal hole; 13 to a lanced and raised portion; 14 to a shouldered portion; 15 to a seat; 16 to a projection for spot welding; 17 to a spot weld and 18 to a gap, respectively.
A brush used in conventional types of brushgear, as illustrated in FIG. 1, comprises the brush arm 1, the brush base and the terminal 3, all of which are made of a thin electrically conductive metal strip and formed integrally, and the brush is three-point supported at the brush base 2 by the first support means 5 and 5 and the second support means 6 provided on the motor case side plate 4, made of a synthetic resin. The brush arm 1 is adapted to make elastic contact with the commutator 7, as shown in FIG. 2. Though not shown in the figure, the terminal 3 is passed through the motor case side plate 4 and led out for external connection.
As described above, it is essential for brushgear to keep the brush arm 1 in good contact with the commutator 7 at all times. In designing a motor, therefore, consideration must be given to how the brushes are securely supported at predetermined positions without loosening, or how vibrations due to the rotation of the commutator are prevented, or how the pushing force applied onto the commutator 7 is selected at a proper level. Particularly, in a small motor where marked improvements have recently been made to ensure the roundness of the commutator, the brushes have been increasingly constructed of thin resilient electrically conductive metal strips to improve the contact properties thereof to ensure positive contact with unwanted grooves on the commutator while minimizing the pushing force of the brush arm 1 onto the commutator 7. The thinner the brushes, the lower becomes the strength of each portion of the brushes. This may result in unstable support of the brush, or unwanted vibrations due to the revolution of the commutator 7. The unwanted vibrations are caused by a pulling or pushing force exerted in the direction shown by arrows in FIG. 2 by the frictional resistance of the brush arm 1 with the revolving commutator 7. That is, when the commutator 7 revolves clockwise, the brush arm 1 is pulled downward, causing the base 2 to warp into a shape shown by dotted lines in the figure, thus causing the brush arm 1 to be pushed downward (the reverse is the case with the counterclockwise revolution of the commutator). And, as the warpage of the brush base 2 exceeds a given limit, the brush arm 1 is rapidly returned to the original state thereof. In this way, during the revolution of the commutator 7, the brush arm 1 repeats the abovementioned warpage and restoration, resulting in vertical movements, that is, unwanted vibrations.
To overcome this problem, various improvements have been made on brushgear. The present applicant, has so far proposed a number of improvements on brushgear. For example, the present applicant has proposed an improvement where a reinforcement 8 is provided by embossing at the bent portion of the brush arm 1 and the brush base 2 and an auxiliary support means 9 is provided to support the reinforcement 8, as illustrated in FIG. 3, so as to prevent the vibrations of the brush arm 1 in the direction shown by arrows in FIG. 2. Furthermore, other improvements have also been proposed, as shown in FIGS. 4 and 5. In the improvement shown in FIG. 4, bent portions 10 and 10' and a reinforcing portion 11 are provided on the brush not only to reinforce the terminal 3 but also to reinforce the rigidity of the whole brush by forming the bent portion 10 integrally with the brush arm 1 and providing a reinforcing portion 11' at the bent portion of the brush arm 1 and the terminal 3. FIG. 5 shows the state where the brush reinforced in the abovementioned manner is securely held in position with a support means provided on the motor case side plate 4. In the support means shown in FIG. 5, the shouldered portion 14 is provided on the internal wall of the terminal hole 12 and caused to engage with the tip of the lanced and raised portion 13 formed by lancing and raising the terminal 3 shown in FIG. 4 to prevent the brush from falling off. Furthermore, the side edge at the foot of the brush arm 1 is supported by the seat 15 to prevent the brush arm 1 from loosening and twisting (Japanese Patent Unexamined Publication No. 71253-82).
In the foregoing, the conventional types of brushgear proposed by the present applicant have been described, referring to FIGS. 1 through 5. In any types of conventional brushgear, brushes are integrally formed using phosphor bronze, beryllium copper or any other material having good electrical conductivity and high resiliency. Phosphor bronze or beryllium copper as used in brushgear is suitable as a material for brush arms requiring resiliency, but not desirable as a material for terminals because of brittleness or soldering difficulty. In general, terminals should preferably be made of copper, brass or any other electrically conductive material which is easy to solder and flexible enough to withstand repeated bendin. Considering this fact, it has been conceived that terminals 3 and brush arms 1 are made of different materials suited for the respective purposes thereof and joined together into one piece by (i) staking with rivets, or (ii) spot welding.
The joining method (i), however, is not desirable because of the increased number of parts and manufacturing steps, while the joining method (ii) involves the difficulty in spot welding phosphor bronze or beryllium copper with copper or brass. In order to manage to spot weld these metals, projections 16 for spot welding have to be provided on the terminal 3, as shown in FIG. 6. FIG. 7 shows the state where the brush thus spot welded is fitted to a brush holding means. In FIG. 7, numeral 17 indicates the spot welded portions between the brush arm 1 and the terminal 3, accompanying a gap 18 in the joint portion. This gap is also found in the case of the joining method (i) using rivets. In this way, these joining methods cannot satisfactorily prevent the unwanted vibrations of the brush arm 1 resulting from the revolution of the commutator 7.